Silicon-based tunable single passband optical filter

ABSTRACT

A tunable optical filter includes a tunable Fabry-Perot (FP) filter, two tunable waveguide Bragg gratings (WBGs) and a 2×2 3-dB coupler. In one embodiment, the WBGs are implemented in a silicon substrate using polysilicon filled trenches. The FP filter is implemented with two silicon nitride trench WBGs with a gap region between them. The FP filter and the WBGs are respectively tuned to transmit and reflect a selected wavelength. A broadband optical signal is propagated into a first port of the coupler. The coupler propagates half of the beam to one WBG and the other half to the other WBG. The WBGs reflect these portions back to the coupler, which then propagates the reflected portions to the FP filter.

FIELD OF THE INVENTION

[0001] Embodiments of invention relate generally to optical devices and,more specifically but not exclusively relate to semiconductor-basedoptical filters.

BACKGROUND INFORMATION

[0002] Transmission bandwidth demands in telecommunication networks(e.g., the Internet) appear to be ever increasing and solutions arebeing sought to support this bandwidth demand. One solution to problemis to use optical networks, where dense wavelength-division-multiplexing(DWDM) technology is used to support the ever-growing demand for higherdata rates. Commonly used optical components include optical filters.

[0003] An optical filter can be implemented in an optical fiber or in aplanar waveguide circuit (PWC). PWC-based optical filters are likely tobe significant in future WDM systems and networks. However, typicalconventional PWC-based optical filters use special materials such asIII-V compound semiconductors (GaAs, InP, AlGaAs, and so on) and LiNiO3or are mechanical such as micro-electro-mechanical (MEM) structures.These approaches tend to be complex and expensive compared tosilicon-based approaches.

[0004] On conventional optical filter uses a Fabry-Perot (FP) filter. Asis well known, FP filters have two reflective surfaces and a cavitybetween. A FP filter allows optical signals of the resonant wavelengthsto pass through, reflecting signals that are not of the resonantwavelengths. However, a conventional FP filter has multiple transmissionpeaks with the distance between peaks referred to as the free spectralrange (FSR). FP filters achieve relatively narrow pass bands, which aredesirable in many optical filter applications, but the multipletransmission peaks may be unsuitable for DWDM applications. The FSR maybe decreased by lengthening the distance between the reflectivesurfaces, but this increases the width of the pass bands. Further,conventional PWC-based FP filters are typically implemented using MEMtechnology or other relatively complex technology. Thus, a conventionalFP filter may not be practical for use in DWDM applications.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Non-limiting and non-exhaustive embodiments of the presentinvention are described with reference to the following figures, whereinlike reference numerals refer to like parts throughout the various viewsunless otherwise specified.

[0006]FIG. 1 is a diagram illustrating a tunable semiconductor-basedsingle passband optical filter, according to one embodiment of thepresent invention.

[0007]FIG. 2 is a diagram illustrating a cross section of a tunablesemiconductor-based WBG depicted in FIG. 1, according to one embodimentof the present invention.

[0008]FIG. 2A is a diagram illustrating a perspective view of thetunable WBG depicted in FIG. 2, according to one embodiment of thepresent invention.

[0009]FIG. 3 is a diagram illustrating a cross section of a tunablewaveguide FP filter depicted in FIG. 1, according to one embodiment ofthe present invention.

[0010]FIG. 3A is a diagram illustrating a perspective view of thetunable FP filter depicted in FIG. 3, according to one embodiment of thepresent invention.

[0011]FIG. 4 is a diagram illustrating the reflection spectrum of thesemiconductor-based WBG and transmission spectrum of the single passbandoptical filter of FIG. 1.

[0012]FIG. 5 is a diagram illustrating a DWDM optical communicationsystem using a tunable passband optical filter according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0013] Embodiments of the present invention are directed to apparatusand systems (and methods thereof) for optical filtering having asemiconductor-based FP filter, two WBGs and a 2×2 3-dB coupler (alsoreferred to herein as a three-dB coupler). The WBGs are tuned to reflectthe desired wavelength to be passed by the optical filter. Amulti-wavelength input signal is provided at a first port (i.e., inputport) of the three-dB coupler. The two WBGs are coupled to second andthird ports of the three-dB coupler so that the input signal when splitby the three-dB coupler (into two portions of substantially equal power)is received by the two WBGs. The two WBGs introduce a ½π phase shiftbetween the split signals. The WBGs reflect the desired wavelength ofthe split signals back to the three-dB coupler. The two optical beamsreflected from WBGs interfere with each other in the three-dB coupler.Consequently, the three-dB coupler propagates almost all the reflectedsignals to the fourth port with almost no reflected light at its inputport. This fourth port is coupled to the FP filter. The FP filter isdesigned with a narrow passband (also referred to herein as “linewidth”)to further filter the combined reflected signal.

[0014] This architecture advantageously allows the FP filter to bedesigned with a relatively small FSR (and therefore more narrowlinewidth) because the WBGs serve to filter out the other wavelengthcomponents of the input signal. For example, the FP filter can bedesigned with a FSR just large enough to avoid passing the sidelobes ofthe reflected signals from the uniform WBGs. Thus, relatively simpleWBGs may be used (e.g., WBGs with uniform gratings) while achieving adesire linewidth for the tunable optical filter. Various embodiments ofthe present invention are described below.

[0015]FIG. 1 illustrates a semiconductor-based tunable optical filter10, according to one embodiment of the present invention. In thisembodiment, tunable optical filter 10 includes a tunablesemiconductor-based Fabry-Perot (FP) filter 11, tunable waveguide Bragggrating (WBGs) 12 ₁ and 12 ₂, and a 2×2 3 dB coupler (also referred toherein as a three dB coupler). Implementations of tunable FP filter 11and tunable WBGs 12 ₁ and 12 ₂ are described below. Three-dB coupler 13can be implemented with any suitable optical coupling device such as,for example, a resonant waveguide coupler or a multi-mode interference(MMI) device.

[0016] The elements of tunable optical filter 10 are interconnected asfollows. One port of three-dB coupler 13 is connected to one end of awaveguide 14, which is coupled to receive an input optical signal at itsother end. In one embodiment, the input optical signal is a signal foruse in a WDM system having wavelengths λ₁, λ₂, λ₃ and λ₄. Another portof three-dB coupler 13 is coupled to tunable FP filter 11 via awaveguide 15. Tunable WBGs 12 ₁ and 12 ₂ are coupled to the tworemaining ports of three-dB coupler 13 via waveguides 16 and 17,respectively. In this embodiment, tunable WBGs 12 ₁ and 12 ₂ areconnected to three-dB coupler 13 so that they receive the input signalwhen it is split by three-dB coupler 13. Further, tunable FP filter 11,tunable WBGs 12 ₁ and 12 ₂, three-dB coupler 13 and waveguides 14-17 areimplemented on a single semiconductor substrate in some embodiments.

[0017] Tunable optical filter 10 can be tuned to pass one of thewavelengths of a multi-wavelength input signal. For example, in theexample embodiment of FIG. 1, tunable optical filter 10 is configured topass wavelength λ₁. Although the following description is directedtoward this “λ₁” example, tunable FP filter 11 and tunable WBGs 12 ₁ and12 ₂ can be tuned to other wavelengths, depending on the application.

[0018] In operation, the multi-wavelength input signal propagates tothree-dB coupler 13 via waveguide 14. In particular, the input signalhas wavelengths λ₁, λ₂, λ₃ and λ₄. Three-dB coupler 13 splits the inputsignal so that about one half of the signal power propagates to tunableWBG 12, via waveguide 16 and the other half of the signal powerpropagates to tunable WBG 122 via waveguide 17. In particular, theportion propagated to tunable WBG 12, has a phase shift of about ½πrelative to the portion propagated to tunable WBG 122 because of thethree-dB coupler.

[0019] Tunable WBGs 12, and 122 are tuned to have a center wavelength ofλ₁, thereby reflecting wavelength λ₁ and passing wavelengths λ₂, λ₃ andλ₄. The reflected λ₁ wavelengths again pass through three-dB coupler 13.The two optical beams reflected from WBGs 12 ₁ and 12 ₂ interfere witheach other in the three-dB coupler. As a result, the three-dB couplerpropagates almost all the reflected signals to the fourth port withalmost no reflected light power at its input port.

[0020] Tunable FP filter 11 is tuned to pass wavelength λ₁. Thus, thereflected λ₁ portions propagating to tunable FP filter 11 are filteredby tunable FP filter 11 to pass a relatively narrow wavelength bandcentered on wavelength λ₁. In one embodiment, tunable FP filter 11 isconfigured to have a FSR larger than the reflection linewidth of tunableWBGs 12 ₁ and 12 ₂ (i.e., the peak and major sidelobes of theirreflection spectrums). Because the passband of tunable WBG 12 ₁ (andtunable WBG 12 ₂) is relative small compared to the entire wavelengthband spanned by wavelengths λ₁, λ₂, λ₃ and λ₄, tunable FP filter 11 canbe configured to have a linewidth that is significantly narrower thanthe linewidth of the WBGs. Thus, tunable optical filter 10 can be usedfor DWDM applications.

[0021]FIG. 2 illustrates an implementation of tunable WBG 12 ₁ (FIG. 1),according to one embodiment of the present invention. Tunable WBG 12 ₂(FIG. 1) is substantially similar. In this embodiment, tunable WBG 12 ₁is formed in a waveguide 20 formed in a semiconductor substrate. Thesemiconductor substrate includes substrate layer 21, a cladding layer 22formed above substrate layer 21, a core layer 23 formed on claddinglayer 22, another cladding layer 24 formed on core layer 23. In oneembodiment, layers 21-24 are formed using silicon on insulator (SOI)technology.

[0022] In addition, several regions 25 are formed in core layer 23 alongwaveguide 20. In some embodiments, regions 25 are filled trenches, withthe fill material having a refractive index different from that of thematerial of core layer 23. For example, in one embodiment, core layer 23is crystalline silicon of a silicon wafer, with regions 25 beingpolysilicon material. In other embodiments, different materials can beused for core layer 23 and regions 25, provided the selected materialshave different refractive indices.

[0023] Waveguide 20 implements an optical path 27, represented in FIG. 2as a double-headed arrow. In this embodiment, regions 25 are formed tobe substantially perpendicular to optical path 27. Regions 25, in thisembodiment, are polysilicon-filled trenches are formed in core layer 23using standard photolithographic and deposition processes. In oneembodiment, the polysilicon is formed in the trenches using a suitabledeposition technique such as, for example, low-pressure chemical vapordeposition (LPCVD). In other embodiments, regions 25 may be formed bydoping regions 25 to alter the regions' refractive indices. A heatingelement 28 is formed on top of waveguide 20 over regions 25. As will bediscussed below, heating element 28 is used to tune WBG 12 ₁ by changingthe temperature (and thus the refractive indices) of the materials nearheating element 28.

[0024] In operation, an optical beam 29 is propagated along optical path27 through waveguide 20. The interfaces between the regions 25 and corelayer 23 in the optical path result in periodic or quasi-periodicperturbations in the effective refractive index along optical path 27.These perturbations cause multiple reflections of portions of opticalbeam 29. When the Bragg condition is satisfied, wavelength components ofoptical beam 29 having a Bragg wavelength will be reflected by WBG 12 ₁(indicated by an arrow 29 _(R) in FIG. 2). Conversely, wavelengthcomponents of optical beam 29 having non-Bragg wavelengths willpropagate through WBG 12 ₁ (indicated by an arrow 29 _(NR) in FIG. 2).

[0025] Tunable WBG 12 ₁ is described in more detail below. Silicon andpolysilicon are example materials provided for explanation purposes andthat other semiconductor materials including III-V semiconductormaterials or the like may be utilized in accordance with the teachingsof the present invention. As shown, a plurality of regions ofpolysilicon regions 25 are disposed in silicon core layer 23 such thatperiodic or quasi-periodic perturbations in an effective index ofrefraction n_(eff) are provided along optical path 27 through core layer23.

[0026] Silicon and polysilicon have effective refractive indices ofn_(Si) and n_(poly), respectively. A relatively small effectiverefractive index difference Δn_(eff) (or n_(poly)−n_(Si)) is provided ateach interface between core layer 23 and regions 25. In one embodiment,Δn_(eff) is approximately within the range of 0.005 to 0.01. Other valueranges for Δn_(eff) may be utilized in other embodiments of the presentinvention and that 0.005 to 0.01 is provided herewith for explanationpurposes.

[0027] In a further refinement, Δn_(eff) can be changed byperforming/controlling an annealing process on the polysilicon ofregions 105. For example, in one embodiment, regions 105 are formed byfilling the trenches with amorphous silicon (α-Si) and then annealingthe α-Si to form polysilicon. The refractive index of the resultingpolysilicon (n_(poly)) can depend on the annealing process. Thus, byappropriately controlling the annealing process to control n_(poly),Δn_(eff) can be controlled.

[0028] As previously described, core layer 23 can be implemented as partof a SOI wafer. In one embodiment, cladding layer 22 is implemented as aburied oxide layer using known SOI processes. As a result, claddinglayer 22 is disposed between silicon core layer 23 and the rest of thesilicon substrate, indicated as substrate layer 21 in FIG. 2.

[0029] In this embodiment, an additional cladding layer 24 is formed oncore layer 23 such that core layer 23 is disposed between claddinglayers 22 and 24. Cladding layer 24 can be formed on the SOI wafer usingstandard deposition or low-temperature oxidation processes. In oneembodiment, cladding layer 24 is an oxide material or the like. In thisembodiment, waveguide 20 is a rib waveguide as shown in FIG. 2A (thecladding layers and heating element are omitted to promote clarity).

[0030] As previously described, there are periodic or quasi-periodicperturbations in the effective index of refraction along optical path 27through waveguide 20. Because of the effective refractive indexdifference Δn_(eff) described above, multiple reflections of opticalbeam 29 occur at the several interfaces between core layer 23 andregions 25 along optical path 27. In this embodiment, a Bragg reflectionoccurs when a Bragg condition or phase matching condition is satisfied.In particular, for uniform Bragg gratings, a Bragg reflection occurswhen the following condition is satisfied:

mλ _(B)=2n _(eff)Λ,  (1)

[0031] where m is the diffraction order, λ_(B) is the Bragg wavelength,n_(eff) is the effective index of the waveguide and Λ is the period ofthe grating.

[0032] To illustrate, FIG. 2 shows a Bragg condition existing for ABequal to λ₁. Optical beam 29 (including wavelengths λ₁, λ₂, λ₃ and λ₄)propagates to WBG 12 ₁ at one end of waveguide 20. Wavelength λ₁ isincluded in optical beam 29 _(R), which reflected back out of waveguide20 by WBG 12, as described above. The remainder of optical beam 29propagates along optical path 27 through waveguide 20 such that theremaining wavelengths (e.g. λ₂, λ₃ and λ₄) are included optical beam 29_(NR), which propagates out the opposite end of waveguide 20.Accordingly, the Bragg wavelength λ₁ is filtered from optical beam 29and directed out of WBG 12 ₁ as optical beam 29 _(R).

[0033] In this embodiment, WBG 12 ₁ is tunable via heating element 28.In one embodiment, heating element 28 is formed from a metallicmaterial. Heating element 28 controls the temperature of core layer 23and regions 25. More particularly, the indices of refraction of thematerials of core layer 23 and regions 25 can vary with temperature.Thus, by controlling the temperature of core layer 23 and regions 25,the Bragg wavelength can be shifted. In applications in which the WBGneed not be tunable, heating element 28 may be omitted.

[0034] In other alternative embodiments (not shown), the Braggwavelength can be tuned by applying a modulated electric field to corelayer 23 and regions 25 to change the effective refractive indices ofcore layer 23 and regions 25. For example, the plasma optical effect asdescribed in U.S. patent application Ser. No. 09/881,218 filed Jun. 13,2001 by Ansheng Liu et al., entitled “Method And Apparatus For Tuning ABragg Grating In A Semiconductor Substrate” can be used.

[0035]FIG. 3 illustrates an implementation of tunable FP filter 11 (FIG.1), according to one embodiment of the present invention. In thisembodiment, tunable FP filter 11 is formed by implementing tworeflectors in a waveguide 30 with a resonator region 31 of length Lbetween them. In this embodiment, the two reflectors are implementedwith two WBGs. The two WBGs serve as reflecting surfaces while thelength of waveguide between the WBGs (i.e., resonator region 31) servesas the FP cavity.

[0036] In this embodiment, FP filter 11 includes WBGs 32 and 33 formedin a waveguide 30 in substantially the same manner as described abovefor WBG 12, (FIG. 2) without the heating element. In addition, in thisembodiment, WBGs 32 and 33 have silicon nitride regions 35 instead ofthe polysilicon regions 25 (FIG. 2) of WBG 12 ₁. Because the largerefractive index difference (˜1.5) between silicon and silicon nitride,a very broad reflection spectrum (˜130 nm) with high reflectivity ofWBGs 32 and 33 can be obtained with a small number of periods (i.e.,regions 35). For example, in one embodiment, the length of the WBGs canbe about twenty microns, with each region being about one micron wide.In other embodiments, different materials can be used for core layer 23and regions 25, provided the selected materials have differentrefractive indices. In this embodiment, tunable FP filter 11 has aheating element 38 disposed over resonator region 31 rather than overthe WBGs as in WBGs 12 ₁ and 12 ₂ (FIG. 2). As in the WBGs, the heatingelement is used to control the temperature (and thereby the refractiveindex) of material below the heating element. In this way, the centerfrequency of tunable FP filter 11 can be controlled. In someembodiments, waveguide 30 is a rib waveguide as shown in FIG. 3A (thecladding layers and heating element are omitted to promote clarity).

[0037]FIG. 4 illustrates the expected spectral responses of tunable FPfilter 11 (FIG. 3) and tunable WBGs 12, and 122 (FIG. 2). Response 41represents the reflection spectrum of tunable WBG 12 ₁ (and WBG 12 ₂).Response 42 represents the transmission spectrum of tunable FP filter11. As shown by response 41, the passband of the reflection spectrum ofthe tunable WBGs is relatively wide. As shown by response 42, thelinewidth of the transmission spectrum of tunable FP filter 11 isrelatively narrow. In addition, the sidelobes of the WBG response areinsignificant at the wavelengths of the adjacent peaks of tunable FPfilter 11 (off the scale in FIG. 4), thereby preventing wavelengthsoutside of the desired wavelength from passing through tunable FP filter11 via these adjacent peaks. Thus, tunable optical filter 10 (FIG. 1)can provide a relatively low cost, easily fabricated solution foroptical filters in DWDM applications.

[0038]FIG. 5 is a diagram illustrating an exemplary opticalcommunication system 50 using a tunable optical filter according to anembodiment of the present invention. In this embodiment, system 50includes an optical add-drop multiplexer (OADM) 52 having tunableoptical filter 54 that is substantially similar to optical filter 10(FIG. 1), and an optical signal source 56. In this embodiment, anoptical fiber 58 connects optical signal source 56 to OADM 52.

[0039] In one embodiment, optical signal source 56 provides an opticalcommunications beam or the like on which data is encoded. In the exampleof FIG. 5, optical signal source 56 includes three optical transmitterunits (not shown) providing optical signals of wavelengths λ₁, λ₂ andλ₃. In this embodiment, DWDM or the like is employed with the opticalbeam such that a different channel is encoded with each of thewavelengths included in the optical beam. For example, the optical beamcan formed by combining the transmitter outputs using an opticalmultiplexer and amplifying the resulting signal using an erbium dopedfiber amplifier (EDFA). The resulting optical beam is propagated to OADM52.

[0040] Tunable optical filter 54 of OADM 52 can then be used to filterout the λ₁ wavelength from the optical beam, as previously describedabove for tunable optical filter 10 (FIG. 1). An optical transmitter canthen add another signal of wavelength λ₁ to the optical beam (λ₂ and λ₃)outputted by tunable optical filter 54 to utilize the λ₁ channel. OtherOADMs (not shown) can be present in system 50. The optical beam can befinally received by a termination unit (not shown) having an opticaldemultiplexer and three optical receivers (one for each of wavelengthsλ₁, λ₂ and λ₃).

[0041] Embodiments of method and apparatus for a tunable optical filterare described herein. In the above description, numerous specificdetails are set forth (such as the materials of substrate 23 and regions25 and 35, tuning mechanisms, three-dB couplers, etc.) to provide athorough understanding of embodiments of the invention. One skilled inthe relevant art will recognize, however, that embodiments of theinvention can be practiced without one or more of the specific details,or with other methods, components, materials, etc. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring the description.

[0042] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

[0043] In addition, embodiments of the present description may beimplemented not only within a semiconductor chip but also withinmachine-readable media. For example, the designs described above may bestored upon and/or embedded within machine readable media associatedwith a design tool used for designing semiconductor devices. Examplesinclude a netlist formatted in the VHSIC Hardware Description Language(VHDL) language, Verilog language or SPICE language. Some netlistexamples include: a behavioral level netlist, a register transfer level(RTL) netlist, a gate level netlist and a transistor level netlist.Machine-readable media also include media having layout information suchas a GDS-II file. Furthermore, netlist files or other machine-readablemedia for semiconductor chip design may be used in a simulationenvironment to perform the methods of the teachings described above.

[0044] Thus, embodiments of this invention may be used as or to supporta software program executed upon some form of processing core (such asthe CPU of a computer) or otherwise implemented or realized upon orwithin a machine-readable medium. A machine-readable medium includes anymechanism for storing or transmitting information in a form readable bya machine (e.g., a computer). For example, a machine-readable medium caninclude such as a read only memory (ROM); a random access memory (RAM);a magnetic disk storage media; an optical storage media; and a flashmemory device, etc. In addition, a machine-readable medium can includepropagated signals such as electrical, optical, acoustical or other formof propagated signals (e.g., carrier waves, infrared signals, digitalsignals, etc.).

[0045] The above description of illustrated embodiments of theinvention, including what is described in the Abstract, is not intendedto be exhaustive or to be limitation to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible, as those skilled in the relevant art willrecognize.

[0046] These modifications can be made to embodiments of the inventionin light of the above detailed description. The terms used in thefollowing claims should not be construed to limit the invention to thespecific embodiments disclosed in the specification and the claims.Rather, the scope is to be determined entirely by the following claims,which are to be construed in accordance with established doctrines ofclaim interpretation.

What is claimed is:
 1. An apparatus, comprising: a semiconductorsubstrate; a waveguide disposed in the semiconductor substrate, thewaveguide having a core; a first reflector and a second reflectordisposed in the waveguide defining a resonator region between the firstand second reflector, the first and second reflectors including aplurality of regions formed in the core of a material having arefractive index different from that of the core, wherein any resonantwavelengths of an optical signal propagated in the waveguide to thefirst and second reflectors propagates through the first and secondreflectors with substantially no loss.
 2. The apparatus of claim 1wherein the core comprises silicon and the material of the plurality ofregions comprises silicon nitride.
 3. The apparatus of claim 1 furthercomprising a heating element disposed proximate the resonator region. 4.The apparatus of claim 1 wherein the semiconductor substrate is part ofa silicon-on-insulator (SOI) wafer, the waveguide having a claddinglayer formed from the insulator of the SOI wafer.
 5. The apparatus ofclaim 3 further comprising another cladding layer formed on thesemiconductor substrate.
 6. The apparatus of claim 2 wherein theresonator region has a length of approximately one hundred microns. 7.The apparatus of claim 2 wherein the first and second reflectors have alength along the waveguide of approximately 20 microns.
 8. The apparatusof claim 1 wherein each region of the plurality of regions has a widthof approximately one micron.
 9. An integrated circuit Fabry-Perot (FP)filter, comprising: a semiconductor layer having a first region; and afirst reflector and a second reflector formed in the semiconductor layerat opposite ends of the first region, the first and second reflectorsincluding trenches formed in the semiconductor layer and filled with amaterial having a refractive index different from that of thesemiconductor layer.
 10. The integrated circuit FP filter of claim 9wherein the semiconductor layer comprises crystalline silicon disposedabove an insulator layer of a silicon-on-insulator (SOI) wafer, and thematerial of the trenches comprises silicon nitride.
 11. The integratedcircuit FP filter of claim 10 wherein the semiconductor layer is a corelayer of a rib waveguide.
 12. An integrated circuit interferometer,comprising: a first waveguide formed in a semiconductor substrate; afirst grating formed in the semiconductor substrate; a second gratingformed in the semiconductor substrate; and a coupler having a firstport, a second port and a third port, the first, second and third portsformed in the semiconductor substrate, wherein the first, second andthird ports are operatively coupled to the first waveguide, the firstgrating and the second grating, respectively, the coupler to cause firstand second portions of an optical signal propagated to the coupler viathe first waveguide to propagate to the first and second gratings,respectively.
 13. The integrated circuit interferometer of claim 12further comprising an optical filter device operatively coupled to afourth port of the coupler.
 14. The integrated circuit interferometer ofclaim 13 wherein the coupler comprises a multimode interference (MMI)device.
 15. The integrated circuit interferometer of claim 13 whereinthe coupler is a resonant waveguide coupler.
 16. The integrated circuitinterferometer of claim 13 wherein the optical filter device comprises aFabry-Perot filter.
 17. The integrated circuit interferometer of claim16 wherein the Fabry-Perot filter includes: a first region formed in thesemiconductor substrate; and a first reflector and a second reflectorformed in the semiconductor substrate at opposite sides of the firstregion, the first and second reflectors including regions formed in thesemiconductor substrate, the regions being of a material having arefractive index different from that of the semiconductor substrate. 18.A wavelength division multiplexed (WDM) system comprising: an opticalsignal source; and a tunable optical filter to receive an optical signalfrom the optical signal source, the optical filter comprising: a couplerdisposed on a semiconductor substrate, the coupler having a first port,a second port, a third port and a fourth port, the coupler to receivethe optical signal at the first port and propagate first and secondportions of the optical signal to the second and third ports,respectively, the first and second portions having substantially equalpower, a first tunable grating disposed on the semiconductor substrateto receive the first portion from the second port of the coupler andreflect a third portion of the first portion having a selectedwavelength back to the coupler, a second tunable grating disposed on thesemiconductor substrate to receive the second portion from the thirdport of the coupler and reflect a fourth portion of the second portionhaving the selected wavelength back to the coupler, and a tunableFabry-Perot filter disposed on the semiconductor substrate to receivereflected signals from the fourth port of the coupler, the Fabry-Perotfilter having a resonant wavelength equal to the selected wavelength.19. The WDM system of claim 18 wherein the tunable Fabry-Perot filterincludes: a first region formed in the semiconductor substrate; and afirst reflector and a second reflector formed in the semiconductorsubstrate at opposite sides of the first region, the first and secondreflectors including regions formed in the semiconductor substrate andfilled with a material having a refractive index different from that ofthe semiconductor substrate.
 20. The WDM system of claim 19 wherein thetunable. Fabry-Perot filter includes a structure disposed proximate tothe first region to selectively alter a refractive index of the firstregion.
 21. The WDM system of claim 20 wherein the structure includes aheating element.
 22. The WDM system of claim 18, wherein the tunableFabry-Perot filter and the first and second tunable gratings are eachpart of a single mode silicon rib waveguide.
 23. A method of fabricatinga Fabry-Perot filter, comprising: forming a planar waveguide in asemiconductor substrate having a core layer; forming a pair ofreflectors in the planar waveguide with a resonant region between thepair of reflectors.
 24. The method of claim 23 wherein forming a pair ofreflectors further comprises forming a plurality of regions in the corelayer of a material having a refractive index different from that of thecore layer.
 25. The method of claim 24, wherein the core comprisessilicon and the plurality of regions comprise silicon nitride.
 26. Themethod of claim 23 further comprising disposing a heating elementproximate the resonant region.
 27. A method, comprising: propagating amultiple wavelength optical signal into a waveguide formed in asemiconductor substrate; splitting the optical signal into first andsecond substantially equal portions to respectively propagate in secondand third planar waveguides formed in the semiconductor substrate;reflecting a selected wavelength of the first portion to propagate inthe second planar waveguide and the selected wavelength of the secondportion to propagate in the propagate in the third planar waveguide;combining the reflected wavelengths from the first and second portionsto propagate in a fourth planar waveguide formed in the semiconductorsubstrate; and filtering the combined reflected wavelengths to transmitthe selected wavelength.
 28. The method of claim 27 wherein reflecting aselected wavelength comprises introducing a ½π phase shift between thefirst and second portions with a three-dB coupler.
 29. The method ofclaim 28 wherein a silicon-based Fabry-Perot filter is used to filterthe combined reflected wavelengths.
 30. The method of claim 28 whereinsilicon-based Bragg gratings are used to reflect the selected wavelengthfrom the first and second portions.